1. Field of the Invention
The invention of the present application relates to burners for large scale industrial applications. Such burners may be adapted for burning gaseous fuels including natural gas. Such burners may also be adapted for burning fuel oil. And in many cases the burners may be adapted for burning both gaseous fuels and fuel oil either alternatively or at the same time. In particular the invention relates to industrial burners which burn fuel gas and/or oil and are specially constructed and engineered for emitting low levels of nitrogen oxide (NOx) and carbon monoxide (CO) air pollution. The invention also relates to the methodology for operating such burners. More particularly the invention relates to a burner and the methodology for operating the same whereby substantial reductions of CO and NOx emissions are achieved relative to existing burners.
2. The Prior Art Background
Many designs exist for delivering fuel and air to a furnace combustion chamber or firebox. Virtually all modern prior art designs are intended to enhance combustion efficiency. In addition, tube metal temperatures and other furnace component limitations must be taken into consideration in designing furnace burners. More recently governmental regulations and social pressures require designers to take into consideration the reduction of CO and NOx emissions.
One of the best of the more recently developed industrial burners is the Todd Variflame No Internal FGR Injection and No External Gas Injection Burner which uses an array of internal poker tubes for delivering fuel and air to a furnace firebox. This system is the subject matter of U.S. Pat. No. 5,860,803 to Schindler et al. which issued on Jan. 19, 1999 (the xe2x80x9c""803 patentxe2x80x9d). The entirety of the disclosure of the ""803 patent is hereby incorporated herein by reference.
In spite of the efforts of many prior art workers in the field, a perfect solution to the CO and NOx emissions problem remains elusive. Some have tried to reduce NOx emissions by recirculating flue gas into the firebox. However, when flue gas is recirculated from a downstream location, the costs associated with providing and forcing such recirculation are substantial.
The present invention provides a device and methodology for efficiently and economically reducing the amount CO and NOx emission from a combustion chamber without substantially effecting thermal efficiency and/or reaction parameters of the same. In particular the invention provides a novel burner design and novel operating methodology which utilizes internal flue gas recirculation and/or external fuel injection in a venturi tube burner system. More particularly, the invention provides a venturi tube burner system which provides swirled primary and straight line secondary combustion air in the venturi tube and straight line tertiary air outside the venturi tube to provide novel effects in the burner flame formed under the above conditions. Preferably the burner includes internal flue gas recirculation and/or external fuel injection.
As a result of extensive research and development conducted by the present inventors, an improved burner design has been developed whereby it is possible to achieve substantial reductions in CO and NOx emissions without substantial loss of burner efficiency. Thus, in accordance with one aspect of the present invention, a novel round burner is provided which comprises a venturi tube positioned to direct a flow of air through the burner and into a combustion zone in a combustion chamber through an entrance in a wall of the combustion chamber. The venturi tube has inlet and outlet ends and a throat located between the inlet and outlet ends. The outlet end has a larger internal diameter than either the inlet end or the throat. The outlet end of the venturi tube is positioned adjacent the entrance to the combustion chamber and the inlet end of the venturi tube is positioned further from the entrance than the outlet end.
The novel burner of the invention also provides a duct system that includes at least one inlet disposed in fluid communication with the combustion zone, and at least one outlet disposed in fluid communication with the throat of the venturi tube. The duct system is arranged and adapted to recirculate flue gas from a location within said combustion chamber adjacent said combustion zone and into said venturi tube at a location adjacent said throat, whereby the recirculated flue gas is inducted into and intermixed with said flow of air at said throat of the venturi tube. Thus, NOx emission reduction may be achieved without the expense of an external flue gas recirculation system.
In another aspect of the invention, the invention provides a round burner which comprises a venturi tube positioned to direct a flow of air through the burner and into a combustion zone in a combustion chamber through an entrance in a wall of the combustion chamber. The novel burner of this aspect of the invention includes a fuel gas injector arrangement including at least one injector nozzle extending through the wall of the combustion chamber at a location adjacent said combustion zone. Such injector nozzle is in fluid communication with the combustion chamber. The injector nozzle is positioned to direct a flow of fuel gas into said combustion chamber at a location in the wall radially outward of and beyond the inner edge of the entrance.
In yet another aspect of the invention, the novel burner may include both the duct system for recirculated flue gas and the fuel gas injector arrangement described above.
In its more specific aspects, the burner of the present invention may include a first fuel gas nozzle that is located in the venturi tube and which is positioned to introduce a supply of fuel gas into the air flowing through the venturi tube. The burner may also include a swirler positioned so that at least a primary portion of the air flow passes therethrough. Ideally the arrangement of the outlet end of the venturi tube and the swirler may be such that a secondary portion of the air flow does not pass through the swirler. Even more ideally, an annular gap may be provided between the outer periphery at the outlet end of the venturi tube and an inner edge of said entrance. Such gap may be positioned to direct a tertiary air flow around the periphery of the venturi tube and through the entrance into said combustion chamber.
Preferably, at least one first fuel gas nozzle may be positioned centrally of the venturi tube adjacent a longitudinal axis thereof and at a location to introduce fuel gas into said primary portion of the flow of air. At least one fuel gas poker nozzle may also be included at a position to introduce fuel gas into said secondary portion of the flow of air.
The burner of the invention may be equipped to burn either fuel gas or oil.
The invention also provides a method for operating a venturi tube equipped round burner of the sort described above. In accordance with this aspect of the invention, the method comprises directing a flow of air through said venturi tube and into a combustion zone in said combustion chamber through said entrance and recirculating flue gas from a location in said combustion chamber adjacent said combustion zone and into the venturi tube at a location adjacent the throat of the venturi tube, whereby said recirculated flue gas is inducted into and intermixed with the combustion air flow at the low pressure throat of the venturi tube.
In another aspect of the invention, the method may comprise directing a flow of air through the venturi tube and into a combustion zone in a combustion chamber through an entrance in a wall of the combustion chamber and injecting a flow of fuel gas into said combustion chamber at a location radially outward and beyond the inner edge of the entrance and adjacent to said combustion zone. Furthermore, the novel method may include both the recirculation of flue gas and external fuel gas injection as described above.
In a more specific sense, the method may include a step of introducing a first supply of fuel gas into said flow of air. The method also may include a step of passing at least a primary portion of said flow of air through a swirler. Even more specifically, the method may be such that a secondary portion of said flow of air does not pass through the swirler.
In another important preferred aspect of the invention, the method may include a step of causing a tertiary air stream to flow around the periphery of the venturi tube, through a gap provided between the large end of the venturi tube and an inner edge of the entrance to the combustion chamber, and on into the combustion zone.
In another preferred aspect of the invention, the method for operating a venturi equipped round burner may include a step of introducing a first supply of fuel gas into the primary portion of the flow of air, and introducing a second separate supply of fuel gas into said secondary portion of the flow of air.